The invention relates to a device and a method for determining the gas permeability of a container.
Nearly all fixedly closed containers have a greater or lesser permeability with respect to gases. In particular synthetic material bottles, for example, comprised of polyethylene terephthalate (PET) tend to release gases, for example CO2, to the outside if their internal pressure is greater than the external pressure. The intention is therefore to decrease the gas permeation through a coating on the inside and/or on the outside of these bottles.
In order to be able to detect the improvements attained through such coatings, it is necessary to define the gas permeability of the bottles before and after the coating. To this end, measuring instruments are required for determining the gas permeability or the permeation.
In order to inspect the tightness of a container, it is already known to generate with the aid of excess pressure in the container the diffusion of gases to the outside (Prospectus of Leybold-Heraeus GmbH, “Industrielle Dichtheitsprüfung”, 1987, pp. 64, 65). In this excess pressure method the test sample is filled with a test gas or a mixture of test gas and air and provided with an encompassing enclosure of known volume. The test gas leaking out through the leaks of the test sample is concentrated in the enclosure and is measured with a sniffing probe after a defined concentration or service life.
Of disadvantage herein is that a complicated and expensive sniffing probe is applied, which includes a mass spectrometer (cf. Wutz, Adam, Walcher, Theorie und Praxis der Vakuumtechnik, Fourth Edition, 1988, pp. 466, 467). With every change of the test gas, the mass spectrometer must also be replaced every time or must be newly calibrated.
Furthermore, a method for measuring the gas permeability of the wall and/or of the closure of three-dimensional enclosure bodies is known, in which the enclosure body to be tested, which contains a gaseous filling at atmospheric pressure, is placed into a measuring chamber and the free space between the enclosure body and the inner measuring chamber is loaded up with filling bodies (DE 26 10 800). Subsequently, an underpressure is generated in the measuring chamber and the time is measured within which a certain pressure rate of ascent interval is run through, and the time required for this represents a measure of the leakage rate of the enclosure body and/or of the closure. The pressure measurement is performed with a Pirani vacuum gauge, which is a resistance manometer whose electric resistance is a function of the gas pressure. Of disadvantage in this Pirani or heat conduction measuring element is that it operates accurately only in a pressure range from 0.01 mbar to 1 mbar and inaccurately in the pressure range from 10 to 1000 mbar and has only minimal resolution for pressures greater than 100 mbar. In this method is additionally of disadvantage that in the measuring chamber an underpressure up to 0.02 Torr (=0.027 mbar=2.7 Pa) must be generated. For this purpose a vacuum pump is required.
It is furthermore known in leak detection technology to detect a leak in a test sample through an excess pressure in the test sample (Wutz, Adam, Walcher: Theorie und Praxis der Vakuumtechnik, Fourth Edition, 1988, p. 483). Herein the test sample with volume V is filled via a valve with test gas until the desired pressure p1 is reached. The valve is subsequently closed and the time interval Δt is measured within which the pressure decreases by Δp1<<p1. The total leakage rate of the test sample is in this case       q    L    =      V    ·                            Δ          ⁢                                           ⁢                      p            1                                    Δ          ⁢                                           ⁢          t                    .      
The detection sensitivity in excess pressure leak detection by measuring the pressure decrease is limited to 1 mbar·l·s−1. However, this value can only be reached when using special difference pressure measuring instruments. Thus, in the case of this leak detection technology, an underpressure is not generated outside of the test sample but rather an excess pressure in the test sample. Since no limited and defined volume exists outside of the test sample, a pressure drop must be measured in the test sample itself. Herein is disadvantageous that with test samples with large volume it takes a long time until there is an onset of a linear pressure decrease.
The invention is based on the task of ascertaining with simple and cost-effective measuring instruments the permeation of containers, in particular of synthetic material containers.
This task is solved according to the present invention.
The invention consequently relates to a device and a method for the determination of the gas permeability of a container, for example of a bottle comprising PET. This container is therein encompassed by an enclosure which hermetically seals the container against the environment. The interspace between the container and the enclosure has a very small volume in comparison to the volume of the container. At the beginning of the determination of the gas permeability this interspace is brought to, for example, atmospheric pressure while into the container is supplied test gas via special inlet lines until this container is at an excess pressure in comparison to the interspace. Through the subsequent diffusion of the test gas through the wall of the container into the interspace, the pressure in the interspace increases. The pressure increase per unit time is a measure of the gas permeability of the container.
The advantage obtained with the invention comprises in particular that by employing simple pressure measuring instruments the permeation in the container can be ascertained independently of the gas selected in each instance as the excess pressure medium. The ascertainment of the permeation, in addition, is carried out relatively fast. Added to this is the fact that improvement of the gas impermeability by coating the container, for example by means of PVD, CVD, or PECVD methods can be detected very quickly. This is in particular of significance in the case of beverage bottles filled with CO2-containing liquids. Without coating such bottles lose approximately 3% of their carbon dioxide per week. It is furthermore of advantage that the invention is not restricted to a specific test gas. Apart from CO2, oxygen, helium or mixtures of gases can therefore also be utilized.